1. Field of the Invention
This invention relates generally to the on-site production of activated carbon material used for removal of vapor phase contaminants from a gas stream. More particularly, this invention relates to the production of activated carbon material for the removal of trace amounts of vapor phase air toxics, such as mercury, from the flue gas of a combustion process, especially from a coal-fired power plant.
2. Description of the Related Art
The 1990 Clean Air Act Amendments, Title III, require major sources of air emissions to limit the discharge of certain chemical species. Certain of these chemical species are categorized as air toxics, and major sources are required to limit emissions to 10 tons per year for any given air toxin. Certain of these species may be present in the flue gas emitted from combustion processes. Therefore, cost-effective methods for controlling emissions of these species are of significant interest to the operators of these processes.
Air toxics and other species regulated by the 1990 Clean Air Act Amendments can be distributed in both the vapor phase and the solid phase in the flue gas from a combustion process. Typically, the air toxics that are concentrated in the solid phase or particulate matter can be effectively removed by the use of a particulate collection device, such as an electrostatic precipitator (ESP) or fabric filter (FF). Air toxics, such as mercury, that are present in the vapor phase are typically in very low concentrations, for example, parts per million, making removal difficult.
On Dec. 16, 2011, the Environmental Protection Agency (EPA) finalized the first ever national standards to reduce mercury and other toxic air pollution from coal and oil-fired power plants. The standards are called Mercury and Air Toxics Standards (MATS). EPA has subsequently released updates on MATS.
According to US EPA, there are about 1,400 coal and oil-fired electric generating units (EGUs) at 600 power plants covered by these standards. They emit harmful pollutants including mercury, non-mercury metallic toxics, acid gases, and organic air toxics including dioxin. Power plants are currently the dominant emitters of mercury (50 percent), acid gases (over 75 percent) and many toxic metals (20-60 percent) in the United States.
Activated Carbon Injection (ACI), ACI with Fabric Filter (FF), or ACI with Electrostatic Precipitators (ESP) are identified by EPA as existing control methods for mercury control. It is a relatively common practice for power plant operators to purchase the activated carbon from a third party supplier. The commercial activated carbon material is typically transported to and stored at the power plant before it is used in the ACI process. The overall costs of the ACI could be a significant burden for the power plant operators. The high costs are in part due to the lengthy processes used in making the AC material in an off-site facility, and are in part due to the transportation to the power plant.
The Electric Power Research Institute (EPRI) and the University of Illinois at Urbana-Champaign (UIUC) patented a technology (U.S. Pat. Nos. 6,451,094 and 6,558,454) that claims to significantly reduce the cost of activated carbon (AC) for controlling mercury from coal-fired power plants. The technology involves the in-situ production of AC at the power plant using the site coal and then direct injection of the freshly produced sorbent into the flue gas to capture mercury. The AC is injected upstream of a particulate control device, such as an existing ESP or baghouse.
U.S. Pat. No. 6,451,094 disclosed methods for removal of air toxics. It has three independent claims (Claims 1, 8 and 15). Claim 1 is by directly injecting and suspending a carbonaceous starting material, such as coal, into a gas stream with air toxics, where the starting material is transformed into activated carbon material, and collecting the activated carbon material that has absorbed the air toxics. The shortcoming of this method is that the process of transforming the starting material into the final activated carbon material also produce volatile organic compounds (VOC) and air toxics such as CO in the gas stream. This process has the potential to reduce mercury, but it produces VOC and CO, which are also regulated pollutants by EPA. Therefore this method has limited practical usage. Claim 8 is by injecting and suspending a carbonaceous starting material, such as coal, into a first gas stream to produce the activated carbon, mixing the first gas stream with a second gas stream with air toxics such as mercury to allow the AC from the first stream to absorb the air toxics in the second stream, and collecting the resulting AC. Again the transformation of the starting material to the activated carbon could produce VOC and CO, and these regulated pollutants are carried into the second stream. Claim 15 also does not address the issues of VOC and CO produced in the formation of starting material into the activated carbon.
U.S. Pat. No. 6,558,454 similarly disclosed methods for removal of air toxics. It has four independent claims (Claims 1, 5, 13 and 17). Claims 1, 5 and 13 are very similar to Claims 1, 8 and 15 of U.S. Pat. No. 6,451,094; while Claim 17 is similar to Claim 1 except the starting carbonaceous material is selected from a group consisting of organic liquids and organic gases. These methods all have similar disadvantages: it did not address the volatiles and CO produced in the process of de-volatilization and activation.
In practice, these methods disclosed in U.S. Pat. Nos. 6,451,094 and 6,558,454 could potentially rely on an existing thermal process (such as the primary boiler) to incinerate the VOC and CO in the flue gas, but such practice requires modification to existing equipment and controls, reduces the flexibility of the power plant operation due to the coupling of AC production system and the existing thermal process of power plant. It is often desirable to de-couple the system for producing the activated carbon from the existing system for power generation since a fully coupled AC production system is very difficult to operate considering issues arising from turndown. It is desirable for the production of activated carbon to be on-site and stand-alone at a power plant in order to use the existing coal supply, rather than in-situ production of activated carbon. On-site and stand-alone production allows the collection and storage of activated carbon for subsequent injection to the pollutant-laden gas stream for the removal of pollutants. In-situ production of activated carbon, which is the essence of U.S. Pat. Nos. 6,451,094 and 6,558,454, does not have such advantages. The in-situ production of activated carbon has to be on-site by definition, but on-site production does not necessarily have to be in-situ. This is an important distinction. A stand-alone activated carbon production system could be designed to fit on mobile trailer units and brought on-site for quick installation in a plug-and-play manner, replacing the commercially purchased activated carbon material in part or in the entirety, with minimal interruption/disturbance to the existing operation of the power plant. Such an activated carbon production system could be operated independent of the operation of the power plant.
In view of the foregoing, there exists a need for an improved method for on-site and stand-alone production of activated carbon for removing vapor phase contaminants from a gas stream.